I found the problem post and deleted it. After that I was able to successfully rebuild posts.

I ran into a second problem when rebuilding private messages. This is the error message. It was consistently repeatable.

Code
Fatal error: Maximum execution time of 30 seconds exceeded in /var/www/html/ubbthreads/libs/html.inc.php on line 858

Anyone have an idea what might be causing this?

The Stovebolt Geek
https://www.stovebolt.com/ubbthreads/ubbthreads.php

Server Information
UBB.threads Version 7.7.5
Release 20201027
Server OS Linux
Server Load 0.16
Web Server Apache/2.4.6
PHP Version 7.4.33
MYSQL Version 5.7.43
Database Size 1.85 GB